The present invention is directed to improving the visual appearance of linear fluorescent lamps, and more particularly, to the elimination of visual striations which may occur in gas discharge lamps. Generally, a gas discharge lamp will have an elongated gas-filled tube having electrodes at each end. A voltage between the electrode accelerates electrons movement. This causes the electrons to collide with gas atoms producing positive ions and additional electrons forming a gas plasma of positive and negative charge carriers. Electrons continue to stream toward the lamp""s anode and the positive ions toward its cathode sustaining an electric discharge in the tube and further heating the electrodes. The electric discharge causes an emission of radiation having a wavelength dependent on the particular fill gas and the electrical parameters of the discharge.
A fluorescent lamp is a gas discharge lamp in which the inner surface of the tube is coated with a fluorescent phosphor. The phosphor is excited by the ultraviolet radiation from the electric discharge and fluoresces, providing visible light.
During operation of a gas discharge lamp, such as a fluorescent lamp, a phenomenon known as striations can occur. Striations are zones of light intensity, appearing as dark bands. This phenomenon can give a lamp an undesirable strobing effect. An example of the striation phenomenon is shown in FIG. 1, which depicts a linear fluorescent lamp 10 employing Krypton added as a buffer gas to improve the efficacy of the lamp. In FIG. 1, lamp 10 has striation zones 12 which appear as the dark bands moving along the length of the lamp. Striations in gas discharge lamps are known to occur in cold applications and in other contexts such as Krypton content lamps.
A variety of theories as to why striations occur have been set forth. For example, in U.S. Pat. No. 5,001,386 to Sullivan, it is stated that striations are believed to occur as a result of high-frequency currents re-enforcing a standing wave of varying charge distribution between the lamp electrodes.
Sullivan attempts to solve the striation problem by injecting a dc component superimposed on top of a driving ac current. A disadvantage to this technique, is the requirement that existing typical high-frequency ballasts in the marketplace must be removed and replaced with a unique ballast capable of injecting the dc bias component. Also, by adding the dc bias it is possible to cause damage to the lamp, by moving mercury in the lamp to one end, creating an unbalanced light output. It has also been suggested that increasing the crest factor in a lamp lighting system will eliminate the usual striations. However, increasing the crest factor may also increase the stress on a lamp, which will lead to a shorter lamp life.
Therefore, it would be beneficial to provide a retrofit or upgrade of existing units which does not require the replacement of typical high-frequency ballasts now in place.
The present invention provides a lighting system powered by a system power source. The lighting system includes a ballast in operative connection with the system power source where the ballast is designed to generate a lamp input signal. A lamp input line is operatively connected to receive the lamp input signal. Further, a gas discharge lamp is in operative connection to the lamp input line configured to receive the lamp input signal. An amplitude modulation circuit is then placed in operative connection to the lamp input line, where the amplitude modulation circuit is configured to periodically modulate amplitudes of the lamp input signal prior to the lamp input signal being received by the gas discharge lamp. Operation of the amplitude modulation circuit results in a periodic amplitude modulation of the lamp input signal and eliminating visual striations otherwise occurring in the lamp.